I was able to work out the solution to my issue. Had to tinker around with it a bit for it to work.
If([Phr NPI]IN('1699857490', '1073861118', '1881961662', '1770648933', '1467471490', '1134372881', '1922035856', '1316205537', '1740275973')) THEN ('PREFERRED') ELSE ('NON-PREFERRED')